1. Field of the Invention
The invention relates to an adjustable length gas spring comprising a housing, which has a central longitudinal axis; a piston rod extended out of the housing concentrically of the central longitudinal axis; a piston, which is mounted on the piston rod and displaceable in the housing, and which divides the housing into a first partial volume and a second partial volume, the piston rod passing through the first partial volume; and a valve for connecting the partial volumes with each other.
1. Background Art
Adjustable length gas springs of the generic type are known for instance from U.S. Pat. No. 3,656,593 or U.S. Pat. No. 5,915,674 as so-called two-tube gas springs, the housings of which consist of an external cylinder and an internal cylinder, between which forms an annular space. A valve is provided at one end of the housing. A piston rod is extended out of the other end, a piston being disposed on its inside end, dividing the space in the interior cylinder into two partial volumes. Furthermore, adjustable length gas springs of the generic type are designed as so-called single-tube gas springs, in which a valve is formed in the piston and can be actuated from outside through the piston rod.
These known adjustable length gas springs are employed substantially as length adjustment elements in adjustable height chair columns. A problem resides in that when the piston rod is inserted to a large extent into the gas spring, with the valve being closed, the ease and convenience that springiness offers to a user sitting on the chair is low, because breakdown of the gas spring may occur in particular when heavyweight persons sit down on the chair.
To solve this problem, it has become known from U.S. Pat. No. 4,318,536, in a two-tube gas spring, to form a valve space in the partial volume turned towards the valve and to join to the piston a closing element which projects towards the valve space, moves sealingly into the valve space when the piston rod has a given position of insertion and separates the valve space from the partial volume. In this case, a marginal condition consists in that, when the valve space is closed by the closing element, the quotient of the cross-sectional surfaces of the piston ring surface and the surface of the closing element exceeds the quotient of the volume of a remaining space and the volume of the valve space. This helps obtain reliable terminal spring stiffening; however, the constructional requirements are considerable.